In recent years, solid-state image sensing elements mainly used in digital still cameras and the like are roughly classified into a CCD (Charge-Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) (e.g., see Japanese Patent Laid-Open Nos. 2002-141488 and 2002-083948).
Principal components of the structure of the CCD will be briefly explained first with reference to FIG. 28.
FIG. 28 is a sectional view of one pixel of a CCD 1000. Referring to FIG. 28, reference numeral 1001 denotes a semiconductor substrate formed of, e.g., silicon; 1002, a photoelectric conversion element including a photodiode; 1003, an oxide film formed on the semiconductor substrate 1001; 1004, three wiring layers which are formed of polysilicon or the like and transmit clock signals used to transfer a charge and the like converted by the photoelectric conversion element 1002; 1006, a light-shielding layer which is formed of tungsten or the like and shields from light a vertical CCD register 1005, used for charge transfer, mainly formed under the wiring layers 1004; 1007, a first protection film which is formed of SiO2 or the like and protects the photoelectric conversion element 1002 and the like from external air (O2, H2O), impurity ions (K+, Na+), and the like; and 1008, a second protection film formed of an SiON-based material or the like. Reference numeral 1009 denotes a flattening layer which is formed of an organic material and reduces the unevenness of the second protection film 1008; and 1010, a microlens for focusing light from an object onto the photoelectric conversion element 1002.
The flattening layer 1009 reduces the unevenness of a principal surface 1011 of the CCD 1000, and also serves to adjust the focal length of the microlens 1010 so as to form a focal point of the microlens 1010 on the photoelectric conversion element 1002. Hence, the thickness of the flattening layer 1009 made up of a transparent photosensitive resin is determined by the curvature of the lens and the refractive index of the lens material.
Principal components of the structure of the CMOS will be briefly described below using FIG. 29.
FIG. 29 is a sectional view of one pixel of a CMOS 1050. Referring to FIG. 29, reference numeral 1051 denotes a silicon substrate (Si substrate), on which a photoelectric conversion unit 1052 serving as a photoelectric conversion element (e.g., a photodiode or the like) is formed. Reference numeral 1054 denotes an insulating interlayer formed of SiO2 or the like; and 1053, a transfer electrode which is formed in the insulating interlayer 1054, and is used to transfer a photo charge generated by the photoelectric conversion unit 1052 to a floating diffusion (FD) unit (not shown). Reference numeral 1055 denotes wiring electrodes which are formed to prevent light from entering portions other than the photoelectric conversion unit 1052 and have a light-shielding effect; 1056, a flattening layer which is formed on a somewhat bumpy surface due to electrodes and wiring layers (not shown) to provide a flat surface; 1057, a color filter of, e.g., red, green, or blue; and 1059, a microlens. The microlens 1059 is formed on the flattening layer 1058. The shape of the microlens 1059 is determined to focus a light beam coming from an image sensing lens (not shown) onto the photoelectric conversion unit 1052.
An image sensing system (zoom mechanism) of a digital camera with the aforementioned solid-state image sensing element will be described below.
FIG. 30 is a schematic sectional view of an image sensing system 1100 of a compact type digital camera. Referring to FIG. 30, reference numeral 1101 denotes a first lens group; 1102, a second lens group; and 1103, a third lens group. The first and second lens groups 1101 and 1102 are movable in the optical axis direction within predetermined ranges for zooming, and the third lens group 1103 is movable in the optical axis direction within a predetermined range for focus adjustment. Reference numeral 1104 denotes an optical low-pass filter; and 1105, an image sensor using a solid-state image sensing element such as a CCD, CMOS, or the like. Reference numeral 1106 denotes a stop, whose aperture size is changed by a driving source 1107.
Reference numeral 1110 denotes a holding member of the first lens group 1101; 1111, a guide pin that guides movement of the first lens group 1101 in the optical axis direction; 1120, a holding member of the second lens group; and 1121, a guide pin that guides movement of the second lens group 1102 in the optical axis direction.
Reference numeral 1130 denotes a cam cylinder which has a cam groove 1131 for moving the first lens group 1101 in the optical axis direction, and a cam groove 1132 for moving the second lens group 1102 in the optical axis direction. The cam cylinder 1130 is movable within a predetermined range in the optical axis direction. Note that the guide pin 1111 cam-fits into the cam groove 1131, and the guide pin 1121 cam-fits into the cam groove 1132. Reference numeral 1133 denotes a guide pin which guides movement of the cam cylinder 1130 in the optical axis direction, and cam-fits into a cam groove 1141 formed in a cam cylinder 1140.
When the cam cylinder 1140 rotates by a driving source (not shown), the cam cylinder 1130 moves in the optical axis direction. As a result, the first and second lens groups 1101 and 1102 move by predetermined amounts in the optical axis direction while being guided by the cam grooves 1131 and 1132 formed in the cam cylinder 1130. With this movement, zooming of the image sensing system 1100 is attained.
Reference numeral 1150 denotes a holding member of the third lens group 1103; and 1160, a holding member of the optical low-pass filter 1104 and image sensor 1105. The holding member 1160 axially supports a motor 1161 to be rotatable. A male screw 1162 is integrally formed on the motor 1161. Since the male screw 1162 is threadably coupled to a female screw 1163 held by the holding member 1150, the holding member 1150 moves along a guide bar (not shown) within a predetermined range in the optical axis direction upon rotation of the motor 1161, i.e., the male screw 1162. In this manner, focus adjustment of the image sensing system 1100 by the third lens group 1103 is attained.
FIG. 31 is a schematic view of a lens-exchangeable digital still camera. FIG. 31 depicts an example of a camera system in which a tele-photo lens 1120 is mounted on a camera body 1200.
The camera body 1200 and tele-photo lens 1220 are coupled via a camera-side mount 1211 and lens-side mount 1221. An electrical circuit such as a lens MPU and the like (not shown) provided to the tele-photo lens 1220 is coupled to an electrical circuit such as a camera MPU and the like (not shown) via a lens-side contact 1222 and camera-side contact 1212.
When a photographer observes an object via a viewfinder, some light components of object light transmitted through the tele-photo lens 1220 are reflected by a quick return mirror 1201 and reach a focusing screen 1202. The object light scattered and transmitted through the focusing screen 1202 is guided to the photographer's eye (not shown) via a pentaprism 1203 and eyepiece 1204.
Also, some light components of the object light are transmitted through the quick return mirror 1201, are reflected by a sub mirror 1205, and are guided to a focus detection unit 1206. The camera MPU calculates a focus adjustment amount of the tele-photo lens 1220 on the basis of an image signal obtained by the focus detection unit 1206, and drives a lens 1223 of the tele-photo lens 1220.
Upon sensing an image, the quick return mirror 1201 and sub mirror 1205 rotate in the direction of the focusing screen 1202, and allow object light transmitted through the tele-photo lens 1220 to be incident on to an image sensor 1208. Since the exit pupil position varies depending on the focal length and the like of an exchangeable lens mounted on the camera body 1200, a light beam that can be received by pixels of, especially, a peripheral portion of the image sensor 1208 changes depending on the exchangeable lens mounted.
Light rays obliquely enter pixels of the periphery of a frame of the image sensor 1208. At this time, as disclosed in Japanese Patent Laid-Open No. 1-213079, if each microlens decenters with respect to the photoelectric conversion unit, it can guide light rays to the photoelectric conversion unit. However, when the condition of the exit pupil of the image sensing lens changes, light rays cannot enter the photoelectric conversion unit and the frame periphery often becomes dark. Such phenomenon conspicuously occurs when the pixel size is reduced. Especially, when an image sensing lens that has a zoom function and focus adjustment function is used, the phenomenon poses severe bottlenecks.
Hence, in an image sensing device that uses an image sensing element comprising an on-chip microlens (Japanese Patent Laid-Open No. 2000-324505), gain control is applied for respective color components of an image signal in accordance with lens information of an exchangeable lens and the distance from the center of an image sensing surface, thus correcting deterioration of sensitivity and variations of hue due to shading and limb darkening. By applying gain control for respective color components using information associated with the exit pupil position of an image sensing lens, shading can be eliminated.
Japanese Patent Laid-Open No. 5-283661 discloses a solid-state image sensing device which includes a light guide between a photo-receiving unit and focusing lens. The light guide of that solid-state image sensing device is formed of a material with a high refractive index, and light that has entered the light guide is guided to the photo-receiving unit while being totally reflected within the light guide, thus improving the focusing characteristics.
Japanese Patent Laid-Open No. 2003-163826 discloses a technique associated with shading correction information of an image sensing system including an exchangeable lens. Vignetting data and exit pupil position data are stored on the exchangeable lens side, and incident-angle dependent data of an image sensing element output are stored on the camera body side, thus realizing shading correction that reflects the characteristics of both the exchangeable lens and camera body.
Japanese Patent Laid-Open No. 8-223587 is an example of disclosure of a technique that pertains to color correction means for preventing a hue change of an image due to chromatic aberration of an on-chip microlens. A change in hue of an image due to a change in size of a focusing spot of the exit pupil projected onto the photoelectric conversion unit of the image sensing element depending on the wavelength of light is eliminated using a color correction means that corrects the ratio of color stimulus values of an image signal in accordance with the exit pupil position of an image sensing lens.
However, the shading correction and color correction techniques disclosed in Japanese Patent Laid-Open Nos. 2000-324505, 2003-163826 and 8-223587 basically electrically correct an image signal on the basis of the exit pupil position of an image sensing lens. However, boosting a signal level to an appropriate level by applying electrical gain is to enhance not only signal components but also noise components, resulting in a low-quality image in which noise is conspicuous on the darkening-corrected peripheral portion.
In case of the conventional compact digital camera, the type of its image sensing system is limited. Such limitation will be explained below using FIG. 32.
FIG. 32 shows a case wherein an object light beam 1061 enters the CMOS 1050 as the conventional solid-state image sensing element at a given tilted incident angle (20° with respect to a central axis 1060 of the microlens 1059 in FIG. 32). In this case, most light components of the object light beam 1061 transmitted through the microlens 1059 do not enter the photoelectric conversion unit 1052. The same applies to the CCD 1000 as the solid-state image sensing element.
That is, there must be a given limitation on an angle that the object light beam 1061 which exits the image sensing system and enters the microlens 1010 or 1059 makes with the central axis of the microlens 1010 or 1059 (to be referred to as an incident angle of an object light beam hereinafter). The object light beam 1061 must enter the microlens at an angle of 10° or less. In case of the image sensing system 1100 described in FIG. 30, the incident angle of the object light beam 1061 falls within the range from 3 to 9 by zooming.
That is, in case of the compact digital camera using the conventional solid-state image sensing element, since its image sensing system is limited to a retrofocus system, the degree of freedom in design of the image sensing system drops, and a size reduction of the image sensing system is disturbed.
In order to solve this problem, even when the compact digital camera or lens-exchangeable camera system is formed using the solid-state image sensing device having the light guide disclosed in Japanese Patent Laid-Open No. 5-283661 above, the light guide cannot often cause total reflection depending on the exit pupil position of the image sensing system of the compact digital camera or the exchangeable lens mounted on the camera. As a result, light cannot be sufficiently collected on the photoelectric conversion unit.
An image capture device disclosed in Japanese Patent Laid-Open No. 2000-324505 performs gain adjustment of an image signal on the basis of exchangeable lens information. However, when limb darkening due to the exchangeable lens is large and the output of the image sensing element is small, gain adjustment must be done at a higher gain. As a result, noise components are also amplified, and a high-quality image signal cannot be obtained.